1. Field of the Invention
This invention relates to a thick film thermal head which is employed to thermally perforate a stencil material to make a stencil to be used in an image forming apparatus such as a stencil printer.
2. Description of the Related Art
As the thermal head used in various image forming apparatuses, there have been known a thin film thermal head and a thick film thermal head. The former is formed by the use of thin film forming technique and the latter is formed by the use of technique other than the thin film forming technique. When perforating a heat-sensitive stencil material by the use of such a thermal head, it is required that adjacent perforations are clearly separated in order to obtain a high printing quality.
A thick film thermal head has been used in a heat-sensitive printing system and a ribbon transfer printing system. The thick film thermal head comprises an insulator substrate such as of ceramic, a plurality of electrodes formed on the substrate and a resistance heater formed on the electrodes. When power is supplied to the electrodes, the resistance heater generates heat from the lower surface thereof in contact with the electrodes and the heat propagates the resistance heater to the upper surface thereof where the resistance heater is brought into contact with a recording medium. In this thermal head, the resistance heater extends across the electrodes and the parts of the resistance heater between the electrodes form resistance heater elements, and each pixel of the image formed by the thermal head becomes larger than the heater element, which results in pixels contiguous to each other.
When the thick film thermal head is used for making a stencil, each of the perforations becomes too large since the heat generated from the lower surface of each of the resistance heater elements spreads over a wide area while the heat propagates to the upper surface of the heat element, and at the same time, it takes a long time for the temperature of the surface of each heater element to reach a perforating temperature, which results in poor response of the thermal head.
Further, in the case of a stencil printer, ink is apt to spread when transferred to the printing paper through the perforations of the stencil and is apt to form printing dots larger than the perforations of the stencil. Accordingly, the perforations of the stencil should be smaller by an amount corresponding to spread of the ink and should be discrete from each other. From this viewpoint, the aforesaid thermal head where heat is generated from the lower surface of the resistance heater elements is not suitable for making a stencil.
In a thermal head having a linear array of resistance heater elements extending in a main scanning direction (in the direction of width of a stencil), though the size of the perforations in the main scanning direction can be reduced by narrowing the intervals at which the electrodes are arranged, it is difficult to reduce the size of the perforations in the sub-scanning direction (the direction in which the stencil is conveyed) due to difficulties in narrowing the width of the resistance heater and influence of heat diffusion.
That is, conventionally, the thick film thermal head is formed by coating resistance heater paste by silk screening on electrodes b formed on a substrate a as shown in FIG. 10. Though the resistance heater paste forms a narrow protrusion as shown by chained line c immediately after coating, it is flattened in the sub-scanning direction with lapse of time as shown by the solid line d. This phenomenon occurs because the resistance heater paste is flowable and there is provided no member for limiting spread of the paste, and makes it difficult to form a narrow resistance heater.
As disclosed, for instance, in Japanese Unexamined Patent Publication No. 63(1988)-165153, there has been proposed a structure in which a heat accumulator layer is formed on an insulating substrate such as of ceramic, a resistance heater is embedded in a groove formed on the heat accumulator layer, and electrodes are formed over the resistance heater for the purpose of making each heat generating area larger than the resistance heater element corresponding thereto and making the pixels contiguous to each other. However this structure of a thermal head is not suitable for making a stencil. That is, when the thermal head is used for thermally perforating a heat-sensitive stencil material, heat generated by each resistance heater element accumulates in the heat accumulator layer and spreads wide, which can result in enlarged or connected perforations. Further the heat accumulator layer deteriorates the speed of response to heat (heat dissipating speed).
There has been known also a thick film thermal head in which a resistance heater in the form of a protrusion is formed on electrodes on a substrate. This type of thermal head is disadvantageous in that paper grounds or resin grounds is peeled off the stencil material by the protruding resistance heater when the stencil material is moved relative to the thermal head during stencil making. The paper grounds or the resin grounds adheres to the surface of the protruding resistance heater and adversely affects stencil making, e.g., prevents the resistance heater from being brought into a close contact with the stencil material and causes the resistance heater to fail in perforating the stencil material.
Due to the difficulties described above, a thin film thermal head has been generally employed for perforating a heat-sensitive stencil material. The thin film thermal head is advantageous in that the resistance heater is of a thin film and accordingly the heat generating area for each resistance heater element can be small, which results in small perforations. However, the thin film thermal head is disadvantageous in that its manufacturing cost is high. That is, the thin film thermal head is manufactured by the use of semiconductor manufacturing technology and expensive apparatuses such as a sputtering apparatus or a vacuum deposition apparatus and high technique are required. At the same time, materials for forming the thermal head are expensive. Further, the semiconductor manufacturing apparatuses are generally for making integral circuits and the like and are not able to produce a large size (e.g., for A2 or larger size) thermal head by one step. Accordingly, a large size thermal head must be produced by incorporating a plurality of small thermal head segments, which gives rise to a problem that heat generation becomes unsatisfactory at junctions between the segments, which can result in white stripes on prints.
To the contrast, the thick film thermal head can be made at low cost, for instance, by screen printing, and can be easily made in a large size.